Search Results: girlfriend caught cheating

I've been in a long distance relationship with my boyfriend for 4 years (we live an hour apart) and we've had our ups & downs. Months ago, a woman we both knew from high school wrote on his fb wall for his birthday saying very friendly & chummy stuff about him on his wall, indicating they had a close friendship. I knew about all his other friends, male/female, but I never knew he associated with her, because he never mentioned her to me.

A week later, I saw incoming text message from this same woman (couldn't read what it said) and I comfronted my boyfriend about it, he just blew it off. He asked me if I wanted him to delete her off his fb page to prove he's faithful. I told him it wouldn't matter, because even if he deleted her to appease me, the truth will come out eventually. He remained friends with her on FB months afterward. So I began watching her page, to see if they're were any signs of a relationship between the two. Yesterday, I discovered, the two of them were no longer friends on FB, which must have happened recently because last week they were friends on FB. In the past, I've caught him red handed, telling people he's single, when in fact he told me, I was his one & only girlfriend in his life, since we been together for the past 4 years.

Im tempted to send her friend request, to see if she'll accept it & I can find any evidence from her page they were more than friends. But I don't feel comfortable doing this, especially since my boyfriend and I are trying to make our relationship better. What should I do?